FIG. 1 is a circuit diagram showing the outline of a conventional inverter apparatus having an overcurrent detector. In FIG. 1, an a.c. current from an a.c. power source 1 is rectified by a rectifier 2, smoothed by a smoothing capacitor 3, and supplied to a voltage type inverter 4. The inverter 4 is constructed of power transistors GT.sub.1 to GT.sub.6 connected in three-phase arrangement. An a.c. output from the inverter is supplied to an a.c. motor 5. The a.c. motor drives, for example, a compressor 5A at the freezing cycle. A frequency command for the inverter 4 is applied to a controller 6. This controller 6 generates an on-signal and off-signal in accordance with the frequency command. In accordance with this on-signal and off-signal, a driver 7 controls the power transistors GT.sub.1 to GT.sub.6. The power transistors GT.sub.1 to GT.sub.6 constituting the inverter 4 have current circulating diodes D.sub.1 to D.sub.6 connected in antiparallel to the transistors. Of the driver 7 for driving the power transistors GT.sub.1 to GT.sub.6, a circuit portion for driving, for example, the power transistor GT.sub.2 is constructed of a pair of serially connected transistors and a reverse bias circuit. The interconnection point between the pair of serially connected transistors is connected to the base of the power transistor GT.sub.2. The reverse bias circuit is connected to the negative side of the pair of serially connected transistors and the emitter of the power transistor GT.sub.2, and is constructed of a parallel circuit of a capacitor C.sub.11 and a serial circuit D.sub.11 of diodes.
A resistor R.sub.1 having low resistance is inserted in a d.c. current path at the negative side to detect an overcurrent of the inverter 4. The resistor R.sub.1 is connected in parallel with a light emitting diode ED which constitutes a photocoupler 8 together with a phototransistor PT. The phototransistor PT is applied with a d.c. power source V.sub.DD via a resistor R.sub.2. These circuit elements constitute an overcurrent detector.
If a current passing through the inverter 4 becomes in excess of a threshold value in accordance with which an overcurrent is determined, the light emitting diode ED of the photocoupler 8 emits light so that the resistance of the phototransistor PT becomes very small. As a result, the potential at the interconnection between the phototransistor PT and the resistor R.sub.2 lowers to an "L" level so that an overcurrent detection signal S.sub.oc is supplied to the controller 6 to protect the inverter 4. Upon reception of the overcurrent detection signal S.sub.oc, the controller 6 performs a protection operation such as lowering an inverter output, or cutting off all the power transistors GT.sub.1 to GT.sub.6.
In the conventional over current detector described above, the light emitting diode ED of the photocoupler 8 generally has a light emission limit voltage of 1.0 resulting from its characteristic. Assuming that the threshold value for detecting an overcurrent is 14.5 A, the resistor R.sub.1 must have a resistance value satisfying the following equation. EQU 1.0/14.5=0.069 .OMEGA. (1)
The power loss by the resistor R.sub.1 is therefore EQU 14.5.sup.2 .times.0.069=14.5 W (2)
It is apparent that considerable heat is generated by this power loss 14.5 W. During a normal operation, although such an excessive current does not flow, there is a power loss near 14.5 W. The operation efficiency is lowered correspondingly, and some countermeasure against heat generation must be provided.